1. Field of the Invention
The present invention relates to an optical reading apparatus, and more particularly, to an optical reading apparatus capable of correcting coma aberration.
2. Description of the Prior Art
Lenses are used for converging, refracting or transmitting light in many optical systems. Ideally, aberration of an image can be reduced by proper lens design. However, the unavoidable aberration, caused by material itself or manufacturing factors in manufacturing processes, causes image deviations. Certain unpredictable factors during operations, i.e. the data surface of the disk having a tilt angle caused by disk misplacement, can also aggravate aberrations in the optical system and thus affect display quality or data access.
Certain major aberrations in an optical system are spherical aberration, coma aberration, astigmatism, curvature of field, etc. Coma aberration in an optical system refers to monochromatic aberration inherent to certain optical designs, or due to imperfection in the lens or a tilted storage medium. Coma aberration results in off-axis images resembling coma tails. Coma aberration represents a spherical aberration of a lens when the incident light is off-axis, and often occurs when the data surface of the storage medium has a large tilt angle with respect to the incident light. Coma aberration is most significant at image rims and can be represented by the following formula:
      coma    ⁢                  ⁢    aberration    =            (                                                  n              2                        -            1                                2            ⁢                                                  ⁢                          n              3                                      *        d        *                              NA            3                    λ                    )        ⁢          (              θ        *                  π          180                    )      
where d is the thickness of the disk substrate, NA is the numerical aperture (NA) of the optical pick-up, λ is the wavelength of incident laser, θ is the tilt angle between the incident laser and the disk substrate, and n is the refractive index of the light in the disk substrate.
With the development of technology, storage media with higher storage capacity is required. FIG. 1 shows a table listing certain specifications of similar storage media available in the market. Compared to the conventional compact disc (CD) and the digital versatile disc (DVD), Blu-ray disk (BD) and the high density digital versatile disc (HD-DVD) have the potential to become the future storage media for their higher storage capacity. BD and HD-DVD drives access data using a laser of a wavelength of 405 nm. Besides, a BD drive further adopts a high NA OPU (optical pick up) for increasing the data capacity and the 0.1 mm thick cover layer.
Coma aberration is inversely proportional to wavelength and proportional to the thickness of the disk substrate and to the third power of NA. Therefore, the tolerances of tilted angle θ in a BD system are largely reduced by using a 0.1 mm thick cover layer. Coma aberration reduces the optical quality more seriously in an HD-DVD system.
A schematic diagram of a prior art optical system 20 disclosed in U.S. Publication 2004/0042356 is shown in FIG. 2. The optical system 20 includes a storage medium 21, an optical pick-up 22, a spindle motor 24, a tilt corrector 26, a land pre-pits (LPP) signal generator 27, a radio frequency (RF) signal generator 28, and a tilt correction calculator 29. An optical sensor disposed on the optical pick-up 22 can transmit signals measured from the storage medium 21 to the LPP signal generator 27 and the RF signal generator 28. The tilt correction calculator 29 generates a tilt correction value based on LPP signals and RF signals respectively received from the LPP signal generator 27 and the RF signal generator 28, and sends the tilt correction value to the tilt corrector 26. Therefore, the optical system 20 can adjust the angle of the optical pick-up 22 when accessing the storage medium 21 for reducing coma aberration. In the optical system 20, an extra tilt corrector is required for correcting aberration, thereby increasing the manufacturing costs and complicating system operations.
A schematic diagram of a prior art optical pick-up 30 disclosed in U.S. Publication 2002/0067672 is shown in FIG. 3. The optical pick-up 30 includes light sources 10 and 12, an optical sensor 14, a conflux prism 31, a collimator lens 32, a grating 33, a polarized beam splitter (PBS) 34, a condenser lens 35, a liquid crystal layer 36, an object lens 37, and an actuator 38. The optical pick-up 30 reduces the aberration when accessing the storage medium 21 using the actuator 38 that includes a focus region and a tracking region. Based on signals measured by the optical sensor 14, the focus region of the actuator 38 adjusts the distance between the object lens 37 and the storage medium 21, and the tracking region of the actuator 38 adjusts the angle of the object lens 37. In the optical pick-up 30, an extra actuator is required for correcting aberration, thereby increasing the manufacturing costs and complicating system operations.
A schematic diagram of a prior art optical pick-up 40 disclosed in U.S. Publication 2004/0125711 is shown in FIG. 4. The optical pick-up 40 includes a light source 42, a spindle motor 43, a PBS 44, an optical sensor 45, a liquid crystal layer 46, a control circuit 47, lenses L1-L3, and indium tin oxide (ITO) electrodes ITO1 and ITO2. In the optical pick-up 40, the control circuit 47 sends correction voltages to the electrodes ITO1 and ITO2 based on signals measured by the optical sensor 45 in an open-loop manner. The aberration, which occurs when the optical pick-up 40 is accessing the storage medium 21, can thus be corrected. However, applying voltages to the electrodes ITO1 and ITO2 cannot improve aberration effectively, and different degrees of aberrations cannot be corrected flexibly.
To reduce coma aberration in the prior art optical systems, an additional control circuit (such as the tilt corrector 26 or the actuator 38) or two ITO electrodes are required for adjusting the angle of the lens. This makes the prior art optical systems more expensive and complicated. However, coma aberration cannot be corrected efficiently and flexibly.